1. Field of the Invention
The invention generally provides for systems and methods of converting to electrical energy the rejected thermal energy of an electrical power generator. More particularly, an embodiment of the invention utilizes a mechanism such as that of a Seebeck device or Stirling engine to convert to electricity the wasted heat of combustion from a combustion-powered electrical power generator.
2. Description of Related Art
In recent years, relatively small, generally portable, electrical power generators powered by petroleum-based, liquid fuel, internal combustion engines have undergone a technological evolution that has significantly increased the efficiency of power generation in situations where the generator load is less than the nominal full load of the generator. That is, traditional constant speed generators have evolved into variable speed generators. Under reduced load requirements traditional constant speed electrical power generators are particularly inefficient because they maintain an engine speed designed for nominal full load output, even though the full load output is not needed. The development of variable speed generators has allowed the engine speed to change in response to the needed electrical output, resulting in significant gains in the overall efficiency of electrical power generators.
A variable speed generator 102 of the prior art is shown in FIG. 1. In general terms, for either a traditional, constant speed generator or a variable speed generator as shown in FIG. 1, the shaft 120 of internal combustion engine 105 drives an alternator 106, which produces alternating current (AC) electrical energy 122 from the rotational motion of the engine. For a variable speed generator 102, however, efficiency is improved over that of a constant speed generator by changing the speed of the engine 105 as the load on the generator 102 changes. The engine speed adjustment is effected in a manner that achieves the maximum possible efficiency from the engine 105 for all loads. By using this variable speed technology the AC voltage produced by the alternator 106 will have varying frequency since the frequency is determined by the number of poles in the alternator and the rotational speed of the shaft. In order to provide a constant AC voltage output from the variable speed generator 102, the alternator voltage output is first converted to a direct current (DC) voltage 124 by a rectifier 107. The DC voltage is then converted to a constant frequency AC voltage output 126 by an electrical power inverter 108. Typical AC voltage frequencies are 50 Hz, 60 Hz and 400 Hz. The variable speed generator, as just described, enables a significant enhancement in generator efficiency for applications in which load levels vary over time.
An example variable speed generator that has achieved significant success is the MEP-831A 3 kW gen-set sold to the U.S. military by Fermont. This generator uses a 7-hp, one cylinder engine that runs at 3600 rpm and is coupled to a permanent magnet alternator paired with an electronic inverter to deliver 120V/60 Hz electrical power. The engine operates using either diesel or JP-8 fuel. (Fuels meeting a “JP” standard are commonly used in avionics applications and are therefore often available to a military that relies on aircraft transportation, which makes very convenient the ability to run the generator on this type of fuel.) This generator is stable against the rigorous environmental demands of military field use. A military generator may need to operate at an elevation as high as 10,000 feet to 14,000 feet or higher, and at temperatures as low as 0° F. to −20° F. or lower and as high as 90° F. to 120° F. or higher. The MEP-831A generator is packaged into a man-portable (as this term is generally used by the U.S. military) unit that weighs about 300 pounds, since portability is important in military applications.
While variable speed generators have provided increased efficiency, the conversion of the chemical energy contained, for example, in petroleum based liquid fuels to electrical energy by a generator that is powered by an internal combustion engine is a relatively low efficiency process. Internal combustion engine (ICE) driven electrical power generators operate at efficiency levels on the order of 20-30%. In operating an ICE driven generator, about 70% of the chemical energy content of the liquid fuel powering the ICE is lost to the environment as thermal energy. In any combustion process used to create electricity through mechanical connections, whether using an internal or external combustion engine based on gaseous, liquid or solid fuels, or any other combustion process, there will be significant energy losses due to the generation of thermal energy that is not captured and converted to electrical energy. Generally, only the mechanical energy that results from the expansion of the combustion product gases is harnessed by the engine 105 to produce the rotational motion that drives the alternator 106.
Whereas an ICE-powered electric energy generator is not designed to utilize thermal energy (which is in fact a waste product of the ICE), other energy conversion mechanisms can be used to produce electrical energy from the rejected thermal energy. One mechanism for generating electrical energy from thermal energy is a thermoelectric device that makes use of the Seebeck effect (a Seebeck device). The Seebeck effect occurs when two dissimilar materials are connected at two separate junctions so as to form an electrical circuit, and the two junctions are maintained at different temperatures. One junction between the two materials is maintained at a higher temperature than is the other junction between the materials. The higher temperature junction is termed the hot side; the lower temperature junction is called the cold side. Under these conditions an electrical current will naturally flow in the circuit. The Seebeck effect has been utilized to design solid state thermoelectric devices, which produce electrical power as a result of exposure to temperature differentials.
Another method of converting thermal energy to electrical energy is through the use of a Stirling engine. Stirling engines feature a system through which work is produced in a cycle of heating and cooling an enclosed fluid (usually a gas) that is connected to at least one piston. Such an engine can utilize any external heat source to convert thermal energy into mechanical energy, e.g., rotational motion, which can then be converted to electrical energy, as was described for the variable speed generator.